Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from flat pieces of material that are substantially transparent, such as quartz or glass, with an opaque layer, such as chrome, on one side. Etched in the opaque layer (e.g., chrome) of the mask is a pattern corresponding to a portion of an electronic circuit design. A variety of different photomasks, including for example, aaPSMs, embedded attenuated phase shift masks and binary photomasks (e.g., chrome-on-glass), are used in semiconductor processing to transfer these patterns onto a semiconductor wafer or other type of wafer.
As shown in FIGS. 1a and 1b, to create an image on a semiconductor wafer 20, a photomask 9 is interposed between the semiconductor wafer 20 (which includes a layer of photosensitive material) and an optical system 22. Energy generated by an energy source 23, commonly referred to as a Stepper, is inhibited from passing through opaque areas of the photomask 9. Likewise, energy from the Stepper passes through the substantially transparent portions of the photomask 9, thereby projecting a diffraction limited, latent image of the pattern on the photomask onto the semiconductor wafer 20. In this regard, the energy generated by the Stepper causes a reaction in the photosensitive material on the semiconductor wafer such that the solubility of the photosensitive material is changed in areas exposed to the energy. Thereafter, the soluble photosensitive material (either exposed or unexposed) is removed from the semiconductor wafer 20, depending upon the type of photolithographic process being used. For example, where a positive photolithographic process is implemented, the exposed photosensitive material becomes soluble and is removed. By contrast, where a negative photolithographic process is used, the exposed photosensitive material becomes insoluble and the unexposed, soluble photosensitive material is removed. After the appropriate photosensitive material is removed, a pattern corresponding to the photomask 9 appears on the semiconductor wafer 20. Thereafter, the semiconductor wafer 20 can be used for deposition, etching, and/or ion implantation processes in any combination to form an integrated circuit.
As circuit designs have become increasingly complex, semiconductor manufacturing processes have become more sophisticated to meet the requirements of these complexities. In this regard, devices on semiconductor wafers have continued to shrink while circuit densities have continued to increase. This has resulted in an increased use of devices packed with smaller feature sizes, narrower widths and decreased spacing between interconnecting lines. For photolithographic processes, resolution and depth of focus (DoF) are important parameters in obtaining high fidelity of pattern reproduction from a photomask to a wafer. However, as feature sizes continue to decrease, the devices' sensitivity to the varying exposure tool wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.) used to write images on a semiconductor wafer has increased, thereby making it more and more difficult to write to an accurate image on the semiconductor wafer. In this regard, as feature sizes continue to decrease, light diffraction effects in the photomask are exacerbated, thereby increasing the likelihood that defects will manifest in a pattern written on a semiconductor wafer. Accordingly, it has become necessary to develop new methods to minimize the problems associated with these smaller feature sizes.
One known method for increasing resolution in smaller feature sizes involves the use of shorter exposure wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.). Shorter exposure wavelengths, however, typically result in a shallower DoF in conventional binary chrome-on-glass (COG) photomasks having smaller feature sizes. In this regard, when the feature size is smaller than the exposure tool wavelength, binary COG photomasks become diffraction limited, thereby making it difficult, if not impossible, to write an accurate image on the semiconductor wafer. Accordingly, phase shifting masks (“PSMs”) have been used to overcome this problem. In this regard, PSMs are known to have properties which permit high resolution while maintaining a sufficient DoF. More particularly, a PSM reduces the diffraction limitation ordinarily associated with a binary COG mask by passing light through substantially transparent areas (e.g., glass, quartz or fused silica) which have either different thickness and/or different refractive indices than an ordinary binary COG mask. As a result, destructive interference is created in regions on the target semiconductor wafer that are designed to see no exposure. Thus, by reducing the impact of diffraction through phase shifting, the overall printability of an image is vastly improved such that the minimum width of a pattern resolved by using a PSM is approximately half the width of a pattern resolved in using an ordinary binary COG mask.
Various types of PSMs have been developed and are known in the art, including aaPSMs as described in co-pending U.S. patent application Ser. No. 10/288,736, filed Nov. 5, 2002 and U.S. patent application Ser. No. 10/391,001 filed Mar. 18, 2003, which are incorporated by reference herein. FIGS. 2a–b illustrate an example of a conventional aaPSM 10. An aaPSM is typically comprised of a layer of opaque material and a substantially transparent substrate which is etched on one side of the opaque features, while not etched on the other side (e.g., etching of the transparent substrate occurs in alternating locations in the substantially transparent substrate). More particularly, as shown in FIGS. 2a–b, the aaPSM 10 includes a substantially transparent layer 13 (e.g., glass, quartz or fused silica) and an opaque layer (e.g., chrome). The opaque layer is etched to form opaque regions 15 and alternating substantially transparent regions 13, as shown in FIG. 2b. The substantially transparent regions 13 are further etched such that the aaPSM 10 has recesses 14 in the substantially transparent layer. In other words, the aaPSM 10 has substantially transparent regions 13 (which are un-etched) that alternate with etched recesses 14 between each opaque region 15, as shown in FIGS. 2a–b. The effect of this structure when placed in a Stepper is to create light intensity of alternating polarity and 180° out of phase, as shown in FIG. 2c. This alternating polarity forces energy transmitted from the Stepper to go to zero, in theory, at opaque regions 15 while maintaining the same transmission of light at the alternating transparent regions 13 and recesses 14. As a result, refraction is reduced through this region. In this regard, in recesses 14, equation (1) is satisfied:d=λ/2(n−1)  (1)where d is film thickness, n is refractive index at exposure wavelength, λ is exposure wavelength. Thus, it is possible to etch smaller features in a semiconductor wafer and use shorter exposure wavelengths. Since the photoresist layer on the semiconductor wafer (FIG. 2d) is insensitive to the phase of the exposed light, the positive and negative exposed regions appear the same, while the zero region in between is clearly delineated. Thus, a sharper contrast between light (e.g., transparent) and dark (e.g., opaque) regions in the resulting photoresist layer of a semiconductor is obtained, thereby making it possible, in theory, to etch a more accurate image onto the semiconductor wafer.
In practice, however, it is difficult to insure as the size of aaPSM continue to get smaller that the etched trenches are formed accurately. Conventional processes used to make aaPSMs etch the photomask to blank a specific depth which is determined by the wavelength of radiation used, as discussed above. Since this depth is significantly less than the photomask substrate thickness, there is no known technique where an optical emission spectrum (OES) could be used to determine the exact and appropriate etch time needed. In addition, there is no additional etching step (referred to as “overetch”) that can be done to address the plasma non-uniformity. Thus, there has been a long felt need for end point detection methods using an OES technique which allows for additional overetch time to adjust for any non-uniformities associated with plasma loading effects due to pattern density on the photomask.
Accordingly, it is an object of the present invention to provide an improved aaPSM that allows for end point detection using an OES technique.
It is a further object of the present invention to provide an improved aaPSM that also allows for additional overetch time to adjust for any non-uniformities associated with plasma loading effects due to pattern density on the photomask.
It is another object of the present invention to solve the shortcomings of the prior art.
Other objects will become apparent from the foregoing description.